Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers, having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformily charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Typically, the fixing device may include a fixing roller heated by a heater, and a pressing roller pressed against the fixing roller to form a fixing nip therebetween through which the recording medium passes. As a recording medium bearing a toner image passes through the fixing nip, the fixing roller and the pressing roller apply heat and pressure to the recording medium to melt and fix the toner image on the recording medium. Thereafter, the recording medium bearing the fixed toner image is discharged from the fixing nip.
As the recording medium passes through the fixing nip, the recording medium draws heat from the fixing roller and the pressing roller, thus cooling them. The heater is designed to take this factor into account. For example, the heater is turned on to heat the fixing roller to a predetermined fixing temperature. However, if the heater heats the fixing roller even after the recording medium is discharged from the fixing nip, the fixing roller may be overheated. To address this problem, operation of the heater is controlled to maintain the temperature of the fixing roller at the predetermined fixing temperature. For example, the heater is turned off after the trailing edge of the recording medium enters the fixing nip. Accordingly, after the recording medium is discharged from the fixing nip, the fixing roller does not overheat.
However, this control method for controlling the heater has a drawback when used in conjunction with a configuration of the fixing device in which the heater is disposed a given distance upstream from the fixing nip in the direction of rotation of the fixing roller. For example, even if the heater is turned off after the trailing edge of the recording medium enters the fixing nip, the heater has already heated an upstream section on the surface of the fixing roller upstream from the fixing nip in the direction of rotation of the fixing roller that will not contact the recording medium and therefore is not used for fixing the toner image on the recording medium, thus wasting power.